The present application claims priority to Swedish Application No. 0003682-2, filed Oct. 12, 2000.
1. Technical Field
The present invention relates to safety belt devices. More specifically, the invention relates to a device for pretensioning and force-limiting a safety belt web joined to a reel mechanism in a vehicle.
2. Background Information
Belt pretensioners with force limiters are presently used for safety belts in motor vehicles in order to take up slack between the belt windings on the spool of the reel mechanism in the event of a collision. At the same time, the pretensioners tighten up the belt against the body of the occupant so as to avoid as much as possible sliding under the belt or jerking, along with a possible accompanying whiplash. This force-limiting function is achieved by controlling the feeding-out of a predetermined length of the belt after pretensioning. This reduces the force between the occupant and the belt web up to the point when the occupant must be caught completely by the belt to prevent his head from hitting interior components such as the steering wheel of the vehicle.
As indicated above, force-limiting after pretensioning occurs when the belt is fed out due to occupant load on the belt caused by vehicle retardation. In a previously known design, force-limiting after pretensioning is achieved by plastic deformation of a torsion element in the reel mechanism. In practical embodiments at desired force levels, the maximum possible feed-out length of the belt is limited by design to approximately 300 mm. However, MADYMO simulations with 95 percentile dummy and 35 miles per hour crash velocity have demonstrated that belt feed-out length should be on the order of approximately 450 mm in order to achieve optimum effect. Accordingly, there is a need for a pretensioner that enables a greater belt feed-out length.
The present invention provides a device for pretensioning and force-limiting a safety belt web joined to a reel mechanism in a vehicle. The device includes a cylinder and a piston which is displaceable in the cylinder and has a piston rod. The piston rod is joined to one side of the piston and extends through an opening in one end wall of the cylinder. The device further includes a motion-transmitting element acting between the piston rod and the reel mechanism. The element is able to achieve rotation of the reel mechanism in the winding-up direction of the belt web upon displacement of the piston in one direction. The device also includes a pyrotechnic charge that, when detonated, causes a pressure increase in a cylinder chamber between the piston and a cylinder end wall, thereby displacing the piston in one direction.
A purpose of the present invention is to achieve a device of the type described above that makes possible controlled feed-out of the desired belt length of approximately 450 mm during the force-limiting phase. This is achieved according to the invention by joining the motion-transmitting element to a ring gear of a planetary gear set so that displacement of the piston in the cylinder results in rotation of the ring gear. The planetary gear set is so disposed and coupled to the reel mechanism that the gear ratio between the ring gear and the reel mechanism is about 1:1 upon rotation of the ring gear in the winding-up direction of the web. The gear set is further disposed and geared up upon rotation in the opposite direction so that movement of the piston over a certain distance in the cylinder corresponds to a several times longer feed-out of the web from the reel mechanism. By utilizing a planetary gear set, the desired belt feed-out length can be simply achieved by adapting the gear up of the planetary gear set to the stroke length of the piston and the desired belt feed-out length.
In a further embodiment of the device according to the invention, the cylinder chamber has an outlet that communicates with a spill valve. The spill valve can be set between various degrees of opening and is controlled by a control unit in response to signals from a sensor for sensing the weight of the occupant. The degree of valve opening determines the resistance against the return stroke of the piston during the force-limiting phase. This implies that the larger the valve opening is, the less will be the resisting force of the belt against the occupant. By regulating the valve opening in relation to the weight of the occupant, the force limitation is regulated relative to the weight of the occupant.
In yet a further embodiment of the device according to the invention, the spill valve can be set to a closed position. The control unit is in this case also coordinated with a positional, so-called roll-over sensor, which senses the position of the vehicle. If the sensor indicates that the vehicle is about to end up upside-down after the pretensioning phase, the control unit will close the valve, which means that the occupant will be held securely against the seat and no belt feed-out can occur. To assure that the piston remains in its position when the valve has been closed, in a preferred embodiment said cylinder chamber is delimited between the side of the piston opposite to the piston rod and the second end wall of the cylinder, the motion-transmitting element being arranged to rotate the reel mechanism in the winding-up direction of the belt when the piston is displaced in the protrusion direction of the piston rod. This provides an expansion space without a through-hole for the piston rod, which would otherwise be a potential path for leakage.